debugger: Fix endless restarts when connecting to TCP adapters over SSH (#34328)

Closes #34323
Closes #34313

The previous PR #33932 introduced a way to "close" the
`pending_requests` buffer of the `TransportDelegate`, preventing any
more requests from being added. This prevents pending requests from
accumulating without ever being drained during the shutdown sequence;
without it, some of our tests hang at this point (due to using a
single-threaded executor).

The bug occurred because we were closing `pending_requests` whenever we
detected the server side of the transport shut down, and this closed
state stuck around and interfered with the retry logic for SSH+TCP
adapter connections.

This PR fixes the bug by only closing `pending_requests` on session
shutdown, and adds a regression test covering the SSH retry logic.

Release Notes:

- debugger: Fixed a bug causing SSH connections to some adapters
(Python, Go, JavaScript) to fail and restart endlessly.

crates/debugger_ui/src/session.rs

@@ -122,7 +122,7 @@ impl DebugSession {
             .to_owned()
     }
 
-    pub(crate) fn running_state(&self) -> &Entity<RunningState> {
+    pub fn running_state(&self) -> &Entity<RunningState> {
         &self.running_state
     }
 

crates/debugger_ui/src/session/running.rs

@@ -1459,7 +1459,7 @@ impl RunningState {
         }
     }
 
-    pub(crate) fn selected_thread_id(&self) -> Option<ThreadId> {
+    pub fn selected_thread_id(&self) -> Option<ThreadId> {
         self.thread_id
     }